Strength members, reinforcing members, and structural members made of metal are used in automobiles and various types of machines A high strength, a low weight, and a small size are demanded of these members. In the past, these members have been manufactured by welding of press worked steel parts, punching of thick steel plates, forging of aluminum alloys, and similar methods. Reductions in weight and size achievable by these manufacturing methods are reaching a limit.
In order to further decrease their weight and size, these members are also manufactured by hydroforming as disclosed in Non-Patent Document 1, for example. Hydroforming manufactures a formed article having a complex shape by supplying a high pressure working fluid to the interior of a metal tube disposed inside dies to expand the metal tube and to deform the metal tube to conform to the inner surface of the dies. Since hydroforming is a type of cold working, it is difficult to use this method to form a material having low ductility such as one having a tensile strength of at least 780 MPa into a complicated shape. Hydroforming usually requires the three steps of bending, preforming, and hydroforming, thereby making the steps relatively complicated. Furthermore, the machinery used for hydroforming is large and relatively expensive.
In Patent Document 1, the present applicant disclosed an apparatus for manufacturing a bent member. FIG. 6 is an explanatory view schematically showing that manufacturing apparatus 0.
A metal tube 1 (in the following explanation, an example will be given of the case in which the metal tube is a steel tube) is supported by a support mechanism 2 so as to be able to move in its axial direction. A feed mechanism 3 feeds the steel tube 1 from the upstream side to the downstream side. The manufacturing apparatus 0 manufactures a bent member 8 by carrying out bending of the steel tube 1 in a position downstream of the support mechanism 2.
Downstream of the support mechanism 2, an induction heating coil 5 locally performs rapid induction heating of the steel tube 1 being fed in its axial direction to a temperature range at which quench hardening is possible (at least the Ac3 point). A water cooling mechanism 6 rapidly cools the steel tube 1 immediately downstream of the induction heating coil 5. As a result, a high temperature portion 1a which moves in the axial direction of the steel tube 1 is locally formed in the steel tube 1 between the induction heating coil 5 and the water cooling mechanism 6. The resistance to deformation of the high temperature portion 1a is markedly lower than the resistance to deformation of other portions.
A movable roller die 4 has at least one set of roll pairs 4a. The roll pairs 4a support the steel tube 1 while feeding it. The movable roller die 4 moves two-dimensionally or three-dimensionally in a region downstream of the water cooling apparatus 6 while supporting the steel tube 1 to apply a bending moment to the high temperature portion 1a. 
The manufacturing apparatus 0 carries out bending of the steel tube 1 with high operating efficiency by simple steps using comparatively inexpensive components 2-6 to manufacture a bent member 8 having a desired shape and a high strength (such as a tensile strength of at least 780 MPa).